'''Cephalopods''' (Class Cephalopoda)are exclusively marine mollusks (Phyllum [[Mollusca]]) with well developed prehensile grasping appediges the encircle the mouth and protrude from a well defined head, and which have a well developed nervous system. All are carnivorous. Other mollusks include [[Gastropoda|gastropods]], snails, [[bivalvia|pelecypods]], clams and other bivalves, and [[Scaphopoda|scaphopods]], tusk shells.

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'''Cephalopods''' (Class Cephalopoda)are exclusively marine mollusks (Phyllum [[Mollusca]]) with well developed prehensile grasping appediges the encircle the mouth and protrude from a well defined head, and which have a well developed nervous system. All are carnivorous. Other mollusks include [[Gastropoda|gastropods]], snails, [[bivalvia|pelecypods]], clams and other bivalves, and [[Scaphopoda|scaphopods]], tusk shells.

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==Biology==

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The name Cephalopoda literally means "head feet" which refers to the cluster of [[arm]]s and/or [[tentacle]]s that project forward from the head, surrounding the mouth. The group includes living [[Coleoidea|coleoids]] (squid, octopods, and cuttlefish) and ''[[Nautilus]]'', and a large number of ancient (mostly [[Paleozoic]] and [[Mesozoic]]) forms. All are active marine predators (although some early types were drifters), able to swim swiftly, and easily competing with fish in the marine habitat. There are 650 living species, but more than 7,500 fossil forms are known (and as in all cases like this this number is obviously a gross underestimate of the real number of cephalopod species that have ever lived through the [[Phanerozoic]] time). Like fish they are equipped with highly developed eyes and other sense organs, include both active swimmers and bottom-dwellers, and in many cases have a streamlined body for more efficient locomotion. Swimming is by rapidly expelling water from the [[mantle cavity]]. The water is forced out through a funnel or [[siphon]], knows as the hyponome, thus driving the animal in the opposite direction. This is the key to the so-called "jet-propulsion" of these animals. The funnel is highly maneuverable and can be directed in almost any direction, allowing motion backwards or forwards. However, the fastest movement is backward escape swimming, with powerful contractions of the mantle ejecting water through the forward facing funnel. A cloud of "ink" can also be ejected as a sort of underwater smoke screen to hide the fleeing animal.

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All cephalopods are carnivorous, feeding primarily on fish, other [[Mollusca|mollusks]], [[Crustacea|crustaceans]], and worms. The head projects into a crown of prehensile arms ranging from 8 in octopus to about 90 in the living nautilus. Cephalopod arms, or tentacles, and used for grasping prey, are a specialized development of the basic molluscan foot. Once the prey is snared it is bitten with strong beak-like jaws and pulled into the mouth by the [[radula]].

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Cephalopods are quite large by molluscan standards (most species being between 6 and 70 cm including tentacles), with the giants of the group - such as the modern day ''[[Architeuthis]]'', the giant squid, with a body length (including tentacles) of up to 20 meters, the Ordovician orthoconic [[Endocerida|endocerid]] ''[[Cameroceras]]'', with a straight shell up to 10 metres in length, and the Cretaceous [[Ammonoidea|ammonoid]] ''[[Parapuzosia seppenradensis]]'', with a coiled shell 3 metres in diameter - the largest invertebrates ever to live, with weights of one to two tons. Such giant cephalopods play or played a similar ecological role of top predator to that of Devonian [[Arthrodira]], Mesozoic [[pliosaur]]s, and Cenozoic [[Odontoceti|toothed whales]].

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Cephalopods have a highly developed nervous system, unequalled among the invertebrates, and correlated with locomotor dexterity and carnivorous lifestyle (predators generally always have larger brains than prey animals). There is a high level of cephalization (development and concentration of sensory and neural centers in the head). The nerve ganglia are concentrated and more or less fused to form a brain that encircles the esophagus. A bundle of giant nerve fibres tied to the [[mantle]] give them very rapid reflexes. They are visual creatures, changing colour to express mood. The eyes of the [[Coleoidea]] are very elaborate, with a retinal structure remarkably like that found in vertebrates. The eye of the giant squid is the largest of any animal - 40 cm across. Nautiloids have smaller and more primitive eyes.

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As with vertebrates, the brain is partitioned into different areas that control particular functions. For example, the brain centres for both forward swimming and closing of the suckers are located in the cerebral ganglia. These animals are most intelligent and highly evolved of the mollusks, indeed they are the most intelligent of all the invertebrates, exhibiting complex patterns of behavior. Octopods can easily be trained to distinguish between classes of objects. See [http://en.wikipedia.org/wiki/Cephalopod_intelligence Cephalopod intelligence]

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==Class Cephalopoda==

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====Basic Divisions====

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===Higher Biological Classification===

*Phyllum Mollusca

*Phyllum Mollusca

** Class Cephalopoda

** Class Cephalopoda

Line 8:

Line 23:

*** Subclass [[ Coleoidea]]

*** Subclass [[ Coleoidea]]

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Most classification divide the Cephalopoda in the three subclasses as shown. Nautiloids are also known as Palcephalopoa, a term which is applied to most but not all members of the subclass. Ammonoids and Coleoids and a few nautiloides come under the term Neocephalopoda. Both terms are based on the morphology and biology of living cephalopods, Palcephalopoda for the presumed primitive ''Nautilus'' and ''Allonautilus'', representing the nautilids and Neocephalopoda for the derived and presumed advanced coleoids.

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Most classification divide the Cephalopoda in the three subclasses as shown. Nautiloids are also known as Palcephalopoda, a term which is applied to most but not all members of the subclass. Ammonoids and Coleoids and a few nautiloides come under the term Neocephalopoda. Both terms are based on the morphology and biology of living cephalopods, Palcephalopoda for the presumed primitive ''Nautilus'' and ''Allonautilus'', representing the nautilids and Neocephalopoda for the derived and presumed advanced coleoids.

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====Systematics====

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The class Cephalopoda has been traditionally divided into three subclasses on the basis of shell structure, or two subclasses on the basis of gills and other soft parts. Both shown here:

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[[Cyrtosoma|CYRTOSOMA]]

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>Shell based classification, used exclusively in paleontology, with three basic subclasses.

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|--+--[[Scaphopoda|SCAPHOPODA]]

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| `-'''CEPHALOPODA'''

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| `- '''[[Nautiloidea]]'''

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| |--[[Plectronocerida]]

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| `-+--[[Ellesmerocerida]]

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| |--[[Endocerida]]

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| |--[[Actinocerida]]

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| |--[[Pseudorthocerida]]

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| |--[[Discosorida]]

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| `--+-[[Orthocerida]]

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| `--'''Neocephalopoda'''

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| |--'''[[Ammonoidea]]'''

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| `--'''[[Coleoidea]]'''

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| |--[[Decapodiformes]]

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| `--[[Octopodiformes]]

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`--[[Gastropoda]]

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==Biology==

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<u>Nautiloidea</u> -- includes primitive forms, all have an external shell and a retrosiphonate siphuncle; represented by modern, tetrabranchiate ''Nautilus'' and ''Allonautilus''.

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The name Cephalopoda literally means "head feet" which refers to the cluster of [[arm]]s and/or [[tentacle]]s that project forward from the head, surrounding the mouth. The group includes living [[Coleoidea|coleoids]] (squid, octopods, and cuttlefish) and ''[[Nautilus]]'', and a large number of ancient (mostly [[Paleozoic]] and [[Mesozoic]]) forms. All are active marine predators (although some early types were drifters), able to swim swiftly, and easily competing with fish in the marine habitat. There are 650 living species, but more than 7,500 fossil forms are known (and as in all cases like this this number is obviously a gross underestimate of the real number of cephalopod species that have ever lived through the [[Phanerozoic]] time). Like fish they are equipped with highly developed eyes and other sense organs, include both active swimmers and bottom-dwellers, and in many cases have a streamlined body for more efficient locomotion. Swimming is by rapidly expelling water from the [[mantle cavity]]. The water is forced out through a funnel or [[siphon]] - the [[hyponome]] - actually a tube-like flap of modified foot, thus driving the animal in the opposite direction. This is the key to the so-called "jet-propulsion" of these animals The funnel is highly maneuverable and can be directed in any direction, allowing motion backwards or forwards. However, the fastest movement is backward escape swimming, with powerful contractions of the mantle ejecting water through the forward facing funnel. A cloud of "ink" can also be ejected as a sort of underwater smoke screen to hide the fleeing animal.

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<u>Ammonoidea</u> -- evolved forms with an external shell and typically a prosiphonate siphuncle; derived from the Nautilioidea but closely related biologically to the dibranchiate coleoids. Became extinct at the end of the Cretaceous.

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All cephalopods are carnivorous, feeding primarily on fish, other [[Mollusca|mollusks]], [[Crustacea]], and worms. The head projects into a crown of prehensile tentacles - ranging from 8 in the octopus to 80 or 90 in the living nautilus. These tentacles are actually a specialized form of the standard molluscan foot, and used for grasping prey. Once the prey is snared it is bitten with strong beak-like jaws and pulled into the mouth by the [[radula]].

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<u>Coleoidea</u> -- shell-less forms and those with an internal vestigial shell represented by living octopods, cuttlefish, squid,and ''Spirula'', all dibranchite. Includes belemnites and other extinct forms.

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Cephalopods are quite large by molluscan standards (most species being between 6 and 70 cm including tentacles), with the giants of the group - such as the modern day ''[[Architeuthis]]'', the giant squid, with a body length (including tentacles) of up to 20 meters, the Ordovician [[Endocerida|endocerid]] [[Nautiloidea|nautiloid]] ''[[Cameroceras]]'', with a straight shell up to 10 metres in length, and the Cretaceous [[Ammonoidea|ammonoid]] ''[[Pachydiscus seppenradensis]]'', with a coiled shell 3 metres in diameter - the largest invertebrates ever to live, with weights of one to two tons. Such giant cephalopods play or played a similar ecological role of top predator to that of [[Devonian]] [[Arthrodira|arthrodire]] [[Placodermi|placoderms]], [[Mesozoic]] [[pliosaur]]s and [[Cenozoic]] [[Odontoceti|toothed whales]].

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>>Anatomical (gill) based classification, used in cephalopod biology.

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Cephalopods have a highly developed nervous system, unequalled among the invertebrates, and correlated with locomotor dexterity and carnivorous lifestyle (predators generally always have larger brains than prey animals). There is a high level of cephalization (development and concentration of sensory and neural centers in the head). The nerve ganglia are concentrated and more or less fused to form a brain that encircles the esophagus. A bundle of giant nerve fibres tied to the [[mantle]] give them very rapid reflexes. They are visual creatures, changing colour to express mood. The eyes of the [[Coleoidea]] are very elaborate, with a retinal structure remarkably like that found in vertebrates. The eye of the giant squid is the largest of any animal - 40 cm across. Nautiloids have smaller and more primitive eyes.

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<u>Tetrabranchiata</u>-- forms with four gills (2 pairs), represented by ''Nautilus'', thought to be primitive. The Palcephalopoda.

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As with vertebrates, the brain is partitioned into different areas that control particular functions. For example, the brain centres for both forward swimming and closing of the suckers are located in the cerebral ganglia. These animals are most intelligent and highly evolved of the mollusks, indeed they are the most intelligent of all the invertebrates, exhibiting complex patterns of behavior. Octopods can easily be trained to distinguish between classes of objects. See [http://en.wikipedia.org/wiki/Cephalopod_intelligence Cephalopod intelligence]

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<u>Dibranchiata</u>-- forms with two gills, considered advanced. Equivalent to the Coleoidea, possibly the Ammonoidea and may be even some Nautilodea

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==Evolutionary History==

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The trouble with the anatomical scheme is that it is impossible to know from shell structure alone alone whether extinct forms had two or four gills, or anything really much about their soft body anatomy. From this it is apparent that the two-fold anatomical classification is inadequate except in modern forms.

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The cephalopods first appeared in the late [[Cambrian]]. The first forms had gently curved shells. During the [[Ordovician]] the group underwent an astonishing evolutionary radiation, possibly due to the new ecological niches made possible by the extinction of [[Anomalocarida|anomalocarids]] at the end of the Cambrian. Some eight new orders appeared. There was tremendous diversity among them. Some had long straight shells, short straight ones, curved, lightly coiled, and tightly coiled ones evolved. The internal structure of the shell differed greatly as well, mostly in the structure of the [[siphuncle]]. Most were probably relatively slow movers, at least compared to today's forms. The largest ones had huge straight shells that reached 3 to 5 or even 10 metres in length. Many of these early forms were [[paraphyletic]], having given rise to newer equal or higher taxa; all are classified within the [[monophyletic]] and valid Subclass Nautiloidea.

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The Subclass Nautiloidea forms a large and diverse, although paraphyletic, group, divisible into phyllogenetically defined superorders. All nautiloids are united by the fact they have simple, adorally concave septa, retrosiphonate septal necks, sutures that are never elaborate, and are descended from a common ancestor, ''Plectronoceras'' They are more than simply cephalopods that are neither ammonoids or coleoids.

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The nautiloid cephalopods remained unchallenged through the Ordovician and [[Silurian]], finally giving way to the large predatory fish of the late Devonian. About this time the ammonoids began to take over from the nautiloids. The ammonoids are rare in the early Devonian, but by the end of the period and the beginning of the [[Carboniferous]] they increase greatly in diversity. During this time, all but two the remaining nautiloid orders die out. The Coleoidea meanwhile make their first appearance in the [[Late Mississippian]] (middle Carboniferous) but remain rare.

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While there is general agreement regarding classification of recent cephalopods, mostly coleoids, based on soft part anatomical features, classification of extinct fossil forms based on such criteria is impossible other than for a few inferences that night be drawn from preserved muscle attachment scars, preserved radula, tentacle imprints, and isolated body impressions. Shell based features on the other hand provide the basis for a reliable, phylogenetically plausible and generally accepted classification of extinct forms.

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The end-Permian extinction exterminated all but a single family of ammonoids. But these adaptable mollusks recovered strongly as the Mesozoic dawned, and the [[Ceratitida|ceratite]] lineage appeared with a great evolutionary radiation during the [[Triassic]]. So successful were these creatures that the Triassic period has been called "The Age of ''[[Ceratites]]''". Over 80 families are known from this time. Another mass extinction at the end of the Triassic saw the demise of the ceratites, along with the last remaining straight-shelled nautiloids (the [[Pseudorthocerida|pseudorthocerids]]). At this time new groups of ammonoids with much more complex sutures ("[[ammonite]]s" in the strict sense) took over.

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As well as ammonites, the squid-like [[Belemnitida|belemnites]], representing the Coleoidea, also underwent a huge evolutionary radiation as the [[Jurassic]] dawned. The first representatives of modern coleoid groups like octopus and squid were other groups of cephalopods that appeared during the Jurassic, but being soft-bodied and only very rarely preserved it is not certain from the fossil record how common they were. But there is no denying that ammonoids, belemnoids, and proto-modern-style coleoids all formed a very significant part of the Jurassic and Cretaceous nektonic marine ecosystems. The ammonoids and belemnoids were to remain highly successful until the end of the [[Cretaceous]], where the same extinction event that killed off the dinosaurs and the other Mesozoic [[megafauna]] also exterminated the Ammonoids. A few belemnoids straggled on until the Eocene, but they were now heavily out-competed by the modern Coleoidea (octopus, squid, cuttlefish, etc).

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'''Palcephalopoda-Neocephalopoda'''

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The Coleoidea remain an important and remarkably successful group of marine invertebrates to this day. Meanwhile, only a few species of pearly nautilus continue as the last survivors of the once important Nautiloidea.

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The <u>Palcephalopoda-Neocephalopoda</u> distinction is based on essential differences between the living nautiloid genera, ''Nautilus'' and ''Allonautilus'', and the huge variety of living coleoids. Living nautiloides are tetrabranchiate (have 4 gills in 2 pairs), are lateradulate (13 elements per radular row), and produce multiple batches of well develovoped offspring from large yolk-rich eggs. The have an external shell, retained from their ancestors but have mutliple finger-like protractable tentacles, probably a derived character. Also their eyes have no lens but operate like a pin-hole camera, again apparently primitive, Coleoids are dibranchiate (have 2 gills), are angusteradulate (9 elements per radular row) and in general produce a single batch of numerous planctonic offspring before dying. They lack an external shell, a obvious derived character but have 8 or 10 muscular suckered and/or hooked tentacles which seems to be a retained primitive trait. On the other hand coleoids have well developed eyes with lenses, sometimes covered by a corneal membrane.

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==Systematics==

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The extent to which these distinctions can be extrapolated into fossil forms is questionable and subject to individual interpretation. Ammonoids for example are thought to be more closely related to coleoids than to nautiloids on the basis of being angueradulate (with 9 radular elements like coleoids) from the few radula found, in spite of having an external shell, and are therefor considered neocephalopods. Paleozoic orthocerids are considered neocephalopods because their protoconch is like that of ammonoids in spite of having an obviously nautiloid phragmocone.

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The class Cephalopoda has been traditionally divided into three subclasses on the basis of shell structure, or two subclasses on the basis of gills and other soft parts. Both shown here:

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==== Alternative Taxonomies====

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Shell based classification, used exclusively in paleontology, with three basic subclasses.

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:'''An Infraclass based taxonomy'''

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'''Nautiloidea''' -- includes primitive forms, all have an external shell and retrosiphonate siphuncle. Modern representatives, Nautilus and Allonautilus are tetrabranchiate.

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The following basically follows Teichert (1988) except that Teichert's two Subclasses Endoceratoidea and Actinoceratoidea have been discarded, since the two orders [[Endocerida]] and [[Actinocerida]] are not so distinct from their contemporaries as to justify such a high taxonomic ranking.

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'''Ammonoidea''' -- evolved forms with an external shell and prosiphonate siphuncle derived from the Nautilioidea. Biologically closely related to the dibranchiates. Became extinct at the end of the Cretaceous.

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'''Infraclass unnamed''' ("Ellesmeroceroidea"? or several infraclasses?)<br>

'''Tetrabranchiata'''-- forms with four gills (2 pairs), represented by Nautilus, thought to be primitive.

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'''Infraclass Ammonoidea''' Agassiz, 1947<br>

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May be equivalent to the Nautiloidea.

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(=Subclass [[Ammonoidea]])

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:Order [[Bactritida]] Shimanskiy

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:Order [[Anarcestida]] Miller & Furnish 1954

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:Order [[Goniatitida]] Hyatt 1884

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:Order [[Clymeniida]] Wedekind 1927

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:Order [[Prolecanitida]] Miller & Furnish 1954

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:Order [[Ceratitida]] Hyatt 1884

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:Order [[Phyllocerida]] Kuhn, 1940

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:Order [[Lytocerida]] Hyatt 1889

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:Order [[Ammonitida]] Agassiz 1847

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:Order [[Ancylocerida]] Wedman 1966

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'''Dibranchiata'''-- forms with two gills, considered advanced. Equivalent to the Coleoidea, possibly the Ammonoidea and mayby even some Nautilodea

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'''Infraclass Coleoidea''' Bather, 1888 <br>

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(=Subclass [[Coleoidea]])

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:Order [[Aulococerida]] Wedman 1966

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:Order [[Boletzkiyida]] Bandel, Reitner & Stürmer 1983

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:Order [[Belemnitida]] Zittel 1885

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:Order [[Belemnoteuthida]] Stolley 1919

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:Order [[Phragmoteuthida]] Jeletzky 1964

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:Order [[Teuthida]] Naef 1916

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:Order [[Sepiida]] Naef 1916

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:Order [[Octopodida]] Leach 1818

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:Order [[Vampyromorpha]] Grimpe 1917 (or in Octopodida)

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The trouble with the anatomical scheme is that it is impossible to know from shell structure alone alone whether extinct forms had two or four gills, or anything really much about their soft body anatomy. From this it is apparent that the two-fold anatomical classification is inadiquate except in modern forms.

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The Subclass Nautiloidea forms a large and diverse, all be it paraphyletic, group, divisable into phyllogenetically defined superorders All nautiloids are united by the fact they have simple, adorally concave septa, retrosiphonate septal necks, sutures that are never elaborate,and are descended from a common ancestor, ''Plectronoceras'' They are more than simply cephalopods that are neither ammonoids or coleoids.

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:'''Superordinal Classification''' from Wade, 1988.

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>Group (Subclass) '''Nautiloidea'''

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While there is general agreement regarding classification of recent cephalopods, mostly coleoids, based on soft part anatomical features, classification of extenct fossil forms based on such criteria is impossible other than for a few inferences that night be drawn from preserved muscle attachment scars, preserved radula, tentacle imprints, and isolated body impressions. Hard part, shell based features on the other hand provide the basis for a relable, phylogenetically plausable and generally accepted classifiation of extinct forms.

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Superorder [[Plectronoceratoidea]]

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: Order: [[Plectronocerida]]

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: Order: [[Yanhecerida]] (or in Plectronocerida)

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: Order: [[Protactinocerida]] (or in Plectronocerida)

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: Order: [[Ellesmerocerida]].

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The following list basically follows Teichert (1988) with the Palcephalopoda/Neocephalopoda hypothesis incorporated; except that Teichert's two Subclasses '''Endoceratoidea''' and '''Actinoceratoidea''' have been discarded, since the two orders [[Endocerida]] and [[Actinocerida]] are probably not so distinct from their contemporaries as to justify such a high taxonomic ranking.

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Superorder Endoceratoidea, = Order: [[Endocerida]]

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===Class Cephalopoda===

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Superorder Actinoceratoidea, = Order: [[Actinocerida]]

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====Subclass: Palcephalopoda====

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The palcephalopoda includes most of the Nautiloidea as originally defined but leaves out the Orthocerida which are combined with the Bactritida, Ammonoidea, and coleoids in the Neocephalopoda. Palcephalopods characteristically have cap-shaped apeces with a cicatrix ore something similar and are retrosiphonte, meaning the septal necks in the siphuncle point to the rear. The apex in neocephalopoda is bulbous and lacks a cicatrix. The only living palcephalopods are ''Nautilus'' and ''Allonautilus''. Both are tetrabranchiate (posses 4 gills), have radula with 9 teeth and 4 marginalia ("Lateradulata") and posses a large number of sticky finger-like arms that lack suckers. Reproduction is what is known as K-selected, with a few large yolk-rich eggs produced a number of times from which large well developed offspring come. Adults are long-lived.

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Superorder Discosoroidea, = Order: [[Discosorida]]

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To what extent these features and characters can be extrapolated into the distant past is unknown. There is some question as to whether the palcephalopoda/neocephalopoda distinction really works with Paleozoic nautloids other that the distinction between different embryonic forms.

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Superorder [[Orthoceratoidea]]

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: Order: [[Orthocerida]]

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: Order: [[Pseudorthocerida]]

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: Order: [[Ascocerida]]

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Superorder [[Nautilitoidea]]

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: Order: [[Tarphycerida]]

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: Order: [[Oncocerida]]

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: Order: [[Nautilida]]

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'''Infraclass unnamed''' ("Ellesmeroceroidea"? or several infraclasses?)

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>Group (subclass '''Dibranchia''')

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Order [[Plectronocerida]] Flower, 1964

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Order [[Yanhecerida]] Chen & Qi, 1979 (or included in Plectronocerida or Ellesmerocerida?)

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Order [[Protactinocerida]] Chen & Qi, 1979 (or included in Plectronocerida or Ellesmerocerida?)

Evolved from Palcephalopoda. Includes a number of lineages with reduced internal shells, and even some (e.g. octopoids and their relatives) that discarded their shell altogether. Radula with seven teeth and two marginalia per row ("Angusteradulata"). Reproduction mostly strategy [[r-selected]] with many small [[plankton]]ic offspring, although some advanced forms (e.g. Octopoda) K-selected with yolk-rich development. Mostly ten arms, which generally possess hooks (belemnites) or adhesive suckers (advanced Coloidea). Siphuncle (in those types that retain their shell) thin and empty. Additional layers on outside of shell. Phragmocone (the shell) originally straight with the siphuncle situated at or near the center. Later the position of the siphuncle shifted to the ventral surface, ([[Bactritida]]), the shell became coiled (Ammonoidea), internal or reduced or absent (Coleoidea). Most extinct forms presumably (like recent forms) with only two gills.